Datasheet

( )
( ) ( )
1 1
OUT1(max) RIPPLE1 CHARGE
L1 peak
2 2
I I I I 10 A 2.5A 0.281A 11.53 A= + ´ + = + ´ + =
( )
OUT1 OUT1
CHARGE
SS
1.2 V 2 220 F 2 10 F 2 3.3 F 1 F
V C
I 0.281A
t 2ms
´ ´ m + ´ m + ´ m + m
´
= = =
RIPPLE1
RIPPLE1
RIPPLE1 RIPPLE(cap)
OUT1 SW
MAX
RIPPLE1 RIPPLE1
I
2.5 A
V
24mV
V V
8 C f
8 458 F 500kHz
ESR 9m
I I 2.5 A
æ ö
æ ö
-
-
ç ÷
ç ÷
-
´ ´
´ m ´
è ø è ø
= = = = W
( )
2
2
OUT1
OUT1(min)
OUT OVER1
I L1
5 0.88 H
C 458 F
V V 1.2 40mV
D ´
´ m
= = = m
´ ´
( )
( )
2
OUT1
OUT1 OUT1 OUT1
UNDER1
OUT1 OUT1 IN OUT1
IN OUT1 OUT1
I L1
I I I L1
V t
C C V V
V V C
D ´
D D D ´
< ´ D = ´ =
-
- ´
( )
D ´
D D D ´
< ´ D = ´ =
´
2
OUT1
OUT1 OUT1 OUT1
OVER1
OUT1 OUT1 OUT1 OUT1 OUT1
I L1
I I I L1
V t
C C V V C
( ) ( )
(
)
( )
( )
( ) ( )
2
2 2 2
2
2
1 1 1
RIPPLE1 OUT1 RIPPLE1
L1 rms L1 avg
12 12 12
I I I I I 10 2.5 10.026 A= + ´ = + ´ = + ´ =
TPS40322
SLUSAF8D –JUNE 2011–REVISED JANUARY 2014 www.ti.com
The RMS current through the inductor is approximated by the equation:
(9)
Output Capacitor Selection (C10 through C16)
The selection of the output capacitor is typically driven by the output transient response requirement. Equation 10
and Equation 11 over-estimate the voltage deviation to account for delays in the loop bandwidth and can be used
to determine the required output capacitance:
(10)
(11)
When V
IN(min)
> 2 x V
OUT1
, use the overshoot equation, V
OVER1
, to calculate minimum output capacitance. When
V
IN(min)
< 2 x V
OUT1
use Equation 11, V
UNDER1
, to calculate minimum output capacitance. In this design example,
V
IN(min)
is much larger than 2 x V
OUT1
so Equation 12 is used to determine the required minimum output
capacitance.
(12)
With a minimum capacitance, the maximum allowable ESR is determined by the maximum ripple voltage and is
approximated by Equation 13.
(13)
Two 220-µF, 4-V, aluminum electrolytic capacitors were chosen for load response requirements. Additionally two
0805 10-µF, X7R, along with two 0603, 3.3-µF X5R, and one 1-µF, X5R ceramic capacitors are selected for low
ESR and high frequency decoupling.
Peak Current Rating of Inductor
With the output capacitance known, it is possible to calculate the charge current during start-up and determine
the minimum saturation current rating for the inductor. The start-up charging current is approximated using
Equation 14.
(14)
(15)
Table 4. Inductor Requirements Summary
PARAMETER VALUE UNITS
L1 Inductance 0.88 µH
IL1_RMS RMS current (thermal rating) 10.026 A
IL1_PEAK Peak current (saturation rating) 11.53 A
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